The present invention relates to storing data to sequential storage media, and more particularly, to improved data storage utilizing more efficient rewrites having larger codeword sizes.
Currently-used linear tape drives which are used to store data sequentially apply product codes for byte-oriented error-correction coding (ECC) to the data prior to storing the data to the tape. These product codes contain two Reed-Solomon (RS) component codes consisting of a C1 row code and a C2 column code. Relatively long (about 1 kB) longitudinal interleaved error correction codewords, also known as codeword interleaves (CWI), are written on tracks of the magnetic medium (e.g., magnetic tape tracks). In current tape drive architectures, CWIs consist of four byte-interleaved RS codewords. During read-while-write, CWIs in a data set (DS) that includes more than a threshold number of errors are rewritten after the writing of the DS that has too many errors therein has been completed. Two CWI sets correspond to a codeword object (CO) set, which represents the minimum amount of data that can be written, or rewritten, on tape using current methodologies. This operating point is selected such that at the beginning of a tape drive's lifetime, the average number of rewritten CWI sets per DS is limited to about a 1% rewrite rate, which corresponds to two rewritten CWI sets per DS. The rewrite rate of 1% is currently reached when a byte error rate at the C1 decoder input is in a range of about 1×10−4 to 1×10−3.
It is possible to operate at a much lower signal-to-noise ratio (SNR) that enables areal density scaling while maintaining a user bit error rate (BER) of 1×10−20 through the use of iterative decoding. However, with currently implemented C1 codes used in conjunction with a conventional rewrite strategy, reducing the SNR would result in a loss of capacity because excess rewrites would be performed during read-while-write processing due to a high C1 error rate that results from the C1 code being used.
Accordingly, improvements are needed to the rewrite strategy to take advantage of all the potential gain of iterative decoding. Specifically, it is desirable to operate the channel at lower signal-to-noise ratio (SNR) values by increasing the symbol error rate at which 1% rewrite rate is achieved by about one order. In other words, we would like the symbol error rate at the C1 decoder input corresponding to 1% rewrite rate to be a number in the range of 1e-3 to 1e-2. Furthermore, currently codewords in CWIs have small error correction capability. They can correct 5 to 6 bytes. This limits the flexibility in the choice of the rewrite error threshold. CWIs with C1 codewords that contain 4 or more errors are rewritten. It would be beneficial to be able to select the rewrite error threshold from a much larger range of numbers. Finally, it is desirable, to increase the rewrite efficiency by minimizing the number of CWI Sets that needs to be rewritten. This invention provides a solution to all these problems.